1. Field of the Invention
The present invention relates generally to technology for programming non-volatile memory devices.
2. Description of the Related Art
Semiconductor memory devices have become more popular for use in various electronic devices. For example, non-volatile semiconductor memory is used in cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices and other devices. Electrical Erasable Programmable Read Only Memory (EEPROM) and flash memory are among the most popular non-volatile semiconductor memories.
One example of a flash memory system uses the NAND structure, which includes arranging multiple transistors in series, sandwiched between two select gates. The transistors in series and the select gates are referred to as a NAND string. FIG. 1 is a top view showing one NAND string. FIG. 2 is an equivalent circuit thereof. The NAND string depicted in FIGS. 1 and 2 includes four transistors 100, 102, 104 and 106 in series and sandwiched between a first select gate 120 and a second select gate 122. Select gate 120 connects the NAND string to bit line 126. Select gate 122 connects the NAND string to source line 128. Select gate 120 is controlled by applying appropriate voltages to control gate 120CG for select gate 120. Select gate 122 is controlled by applying the appropriate voltages to control gate 122CG of select gate 122. Each of the transistors 100, 102, 104 and 106 includes a control gate and a floating gate. For example, transistor 100 has control gate 100CG and floating gate 100FG. Transistor 102 includes control gate 102CG and a floating gate 102FG. Transistor 104 includes control gate 104CG and floating gate 104FG. Transistor 106 includes a control gate 106CG and a floating gate 106FG. Control gate 100CG is connected to word line WL3, control gate 102CG is connected to word line WL2, control gate 104CG is connected to word line WL1, and control gate 106CG is connected to word line WL0.
FIG. 3 provides a cross-sectional view of the NAND string described above. As depicted in FIG. 3, the transistors (also called cells or memory cells) of the NAND string are formed in p-well region 140. Each transistor includes a stacked gate structure that consists of the control gate (100CG, 102CG, 104CG and 106CG) and a floating gate (100FG, 102FG, 104FG and 106FG). The floating gates are formed on the surface of the p-well on top of an oxide or other dielectric composite film. The control gate is above the floating gate, with an oxide or other isolating dielectric layer separating the control gate and floating gate. Note that FIG. 3 appears to depict a control gate and floating gate for transistors 120 and 122. However, for transistors 120 and 122, the control gate and the floating gate are electrically connected together. The control gates of the memory cells (100, 102, 104, 106) form the word lines. N+ doped layers 130, 132, 134, 136 and 138 are shared between neighboring cells whereby the cells are connected to one another in series to form a NAND string. These N+ doped layers form the source and drain of each of the cells. For example, N+ doped layer 130 serves as the drain of transistor 122 and the source for transistor of 106, N+ doped layer 132 serves as the drain for transistor 106 and the source for transistor 104, N+ doped region 134 serves as the drain for transistor 104 and the source for transistor 102, N+ doped region 136 serves as the drain for transistor 102 and the source for transistor 100, and N+ doped layer 138 serves as the drain for transistor 100 and the source for transistor 120. N+ doped layer 126 connects to the bit line for the NAND string, while N+ doped layer 128 connects to a common source line for multiple NAND strings.
Note that although FIGS. 1–3 show four memory cells in the NAND string, the use of four transistors is only provided as an example. A NAND string can have less than four memory cells or more than four memory cells. For example, some NAND strings will include eight memory cells, 16 memory cells, 32 memory cells, etc. The discussion herein is not limited to any particular number of memory cells in a NAND string.
A typical architecture for a flash memory system using a NAND structure will include several NAND strings. For example, FIG. 4 shows three NAND strings 202, 204 and 206 of a memory array having many more NAND strings. Each of the NAND strings of FIG. 4 includes two select transistors and four memory cells. For example, NAND string 202 includes select transistors 220 and 230, and memory cells 222, 224, 226 and 228. NAND string 204 includes select transistors 240 and 250, and memory cells 242, 244, 246 and 248. Each string is connected to the source line by its select transistor (e.g. select transistor 230 and select transistor 250). A selection line SGS is used to control the source side select gates. The various NAND strings are connected to respective bit lines by select transistors 220, 240, etc., which are controlled by select line SGD. In other embodiments, the select lines do not necessarily need to be in common. Word line WL3 is connected to the control gates for memory cell 222 and memory cell 242. Word line WL2 is connected to the control gates for memory cell 224 and memory cell 244. Word line WL1 is connected to the control gates for memory cell 226 and memory cell 246. Word line WL0 is connected to the control gates for memory cell 228 and memory cell 248. As can be seen, each bit line and the respective NAND string comprise the columns of the array of memory cells. The word lines (WL3, WL2, WL1 and WL0) comprise the rows of the array. Each word line connects the control gates of each memory cell in the row. For example, word line WL2 is connected to the control gates for memory cells 224, 244 and 252.
Each memory cell can store data (analog or digital). When storing one bit of digital data, the range of possible threshold voltages of the memory cell is divided into two ranges which are assigned logical data “1” and “0.” In one example of a NAND type flash memory, the threshold voltage is negative after the memory cell is erased, and defined as logic “1.” The threshold voltage after a program operation is positive and defined as logic “0.” When the threshold voltage is negative and a read is attempted by applying 0 volts to the control gate, the memory cell will turn on to indicate logic one is being stored. When the threshold voltage is positive and a read operation is attempted by applying 0 volts to the control gate, the memory cell will not turn on, which indicates that logic zero is stored. A memory cell can also store multiple levels of information, for example, multiple bits of digital data. In the case of storing multiple levels of data, the range of possible threshold voltages is divided into the number of levels of data. For example, if four levels of information are stored, there will be four threshold voltage ranges assigned to the data values “11”, “10”, “01”, and “00.” In one example of a NAND type memory, the threshold voltage after an erase operation is negative and defined as “11.” Positive threshold voltages are used for the states of “10”, “01”, and “00.”
Relevant examples of NAND type flash memories and their operation are provided in the following U.S. Patents/Patent Applications, all of which are incorporated herein by reference: U.S. Pat. No. 5,570,315; U.S. Pat. No. 5,774,397, U.S. Pat. No. 6,046,935, U.S. Pat. No. 6,456,528 and U.S. patent application Ser. No. 09/893,277 (Publication No. U.S. 2003/0002348).
When programming a flash memory cell, a program voltage is applied to the control gate and the bit line is grounded. Electrons from the p-well are injected into the floating gate. When electrons accumulate in the floating gate, the floating gate becomes negatively charged and the threshold voltage of the cell is raised. To apply the program voltage to the control gate of the cell being programmed, that program voltage is applied on the appropriate word line. As discussed above, that word line is also connected to one cell in each of the other NAND strings that utilize the same word line. For example, when programming cell 224 of FIG. 4, the program voltage will also be applied to the control gate of cell 244 because both cells share the same word line. A problem arises when it's desired to program one cell on a word line without programming other cells connected to the same word line, for example, when it's desired to program cell 224 and not cell 244. Because the program voltage is applied to all cells connected to a word line, an unselected cell (a cell that is not to be programmed) connected to the word line, especially a cell adjacent to the cell selected for programming, may become inadvertently programmed. For example, cell 244 is adjacent to cell 224. When programming cell 224, there is a concern that cell 244 might unintentionally be programmed. The unintentional programming of the unselected cell on the selected word line is referred to as “program disturb.”
Several techniques can be employed to prevent program disturb. In one method known as “self boosting,” the unselected bit lines are electrically isolated and a pass voltage (e.g. 10 volts) is applied to the unselected word lines during programming. The unselected word lines couple to the unselected bit lines, causing a voltage (e.g. eight volts) to be impressed in the channel and source/drain regions of the unselected bit lines, thereby reducing program disturb. Self boosting causes a voltage boost to exist in the channel which lowers the voltage across the tunnel oxide and hence reduces program disturb.
A NAND string is typically (but not always) programmed in sequence from the source side to the drain side, for example, from memory cell 228 to memory cell 222. When the programming process is ready to program the last (or near the last) memory cell of the NAND string, if all or most of the previously programmed cells on the string being inhibited (e.g. string 204) were programmed, then there is negative charge in the floating gates of the previously programmed cells. Because of this negative charge on the floating gates, the boosting potential doesn't get high enough and there still may be program disturb on the last few word lines. For example, when programming cell 242, if cells 248, 246 and 244 were programmed, then each of those transistors (244, 246, 248) have a negative charge on their floating gate which will limit the boosting level of the self boosting process and possibly allow program disturb on cell 242.
The problem discussed above with self boosting has been addressed by two other schemes: Local Self Boosting (“LSB”) and Erased Area Self Boosting (“EASB”). Both LSB and EASB attempt to isolate the channel of previously programmed cells from the channel of the cell being inhibited. For example, if cell 224 of FIG. 4 is being programmed, LSB and EASB attempt to inhibit programming in cell 244 by isolating the channel of cell 244 from the previously programmed cells (246 and 248). With the LSB technique, the bit line for the cell being programmed is at ground and the bit line of the string with the cell being inhibited is at Vdd. The program voltage Vpgm (e.g. 20 volts) is driven on the selected word line. The word lines neighboring the selected word line are at zero volts and the remaining non-selected word lines are at Vpass. For example, looking at FIG. 4, bit line 202 is at zero volts and bit line 204 is at Vdd. Drain select SGD is at Vdd and source select SGS is at zero volts. Selected word line WL2 (for programming cell 224) is at Vpgm. Neighboring word lines WL1 and WL3 are at zero volts, and other word lines (e.g. WL0) are at Vpass.
EASB is similar to LSB with the exception that only the source side neighbor word line is at zero volts. For example, WL1 would be at zero volts while WL3 would be at Vpass. In one embodiment, Vpass is 7–10 volts. If Vpass is too low, boosting in the channel is insufficient to prevent program disturb. If Vpass is too high, unselected word lines will be programmed, also a type or form of disturb.
While LSB and EASB provide an improvement over self boosting, they also present a problem that depends on whether the source side neighbor cell (cell 246 is the source side neighbor of cell 244) is programmed or erased. If the source side neighbor cell is programmed, then there is a negative charge on the floating gate of that source side neighbor cell. Zero volts are applied to the control gate. Thus, there is a highly reverse biased junction under the negatively charged gate which can cause Gate Induced Drain Leakage (GIDL). GIDL involves electrons leaking into the boosted channel. GIDL occurs with a large bias in the junction and a low or negative gate voltage, which is precisely the case when the source side neighbor cell is programmed and the drain junction is boosted. GIDL will cause the boosted voltage to leak away prematurely, resulting in a programming error. GIDL is more severe with the abruptly and highly doped junctions, which are required as cell dimensions are scaled. If the leakage current is high enough, the boosting potential in the channel region will go down and there can be program disturb. The closer the word line being programmed is to the drain, the less charge is present in the boosted junction. Thus, the voltage in the boosted junction will drop quickly, causing program disturb.
If the source side neighbor memory cell is erased, then there is positive charge on the floating gate and the threshold voltage of the transistor will likely be negative. The transistor may not turn off even when zero volts is applied to the word line. If the memory cell is on, then the NAND string is not operating in EASB mode. Rather that string is operating in self boosting mode, and self boosting mode has the problems discussed above. This scenario is most likely if other source side cells are programmed, which limits source side boosting. This issue is most problematic with shorter channel lengths.
In addition to the identified problems of each of the aforementioned prior art techniques for reducing or preventing program disturb, the amount of pass or boosting voltage that is applied to unselected world lines has an upper and lower limit. If the pass voltage is below a certain level, insufficient boosting of the channel will occur and programming of an addressed memory cell that is to be fully inhibited can occur. Program disturb experienced by memory cells connected to a word line selected for programming is this manner is often referred to as “on-row” program disturb. If the pass voltage applied to an unselected word line is raised too high, a programming disturb condition can be induced on memory cells connected to the unselected word line that are part of a NAND string having a bit line biased to enable programming (e.g., sitting at 0 volts). That is, application of the pass voltage to a memory cell whose bit line is targeted for programming can lead to programming of that memory cell. Program disturb experienced by memory cells connected a word line receiving a pass voltage is often referred to as “off-row” program disturb.
Thus, there is a need for a better mechanism to prevent program disturb.